CN109314253B

CN109314253B - Method for preparing supported catalyst material for fuel cell

Info

Publication number: CN109314253B
Application number: CN201780040102.8A
Authority: CN
Inventors: T.格拉夫; T.施拉特
Original assignee: Audi AG; Volkswagen AG
Current assignee: Audi AG; Volkswagen AG
Priority date: 2016-06-30
Filing date: 2017-06-26
Publication date: 2022-03-01
Anticipated expiration: 2037-06-26
Also published as: KR20190024902A; CN109314253A; KR102266661B1; EP3479429A1; EP3479429B1; WO2018001930A1; US20190157686A1; US11489167B2; JP6807964B2; DE102016111981A1; JP2019519080A

Abstract

The present invention relates to a method for preparing a supported catalyst material (20) for fuel cell electrodes and a catalyst material (20) that can be prepared by the method. In the method, a carbide-forming substance is first vapor-deposited on the carbon-based support material (21) to produce a carbide-containing layer (22), and then a catalytically active noble metal or alloy thereof is vapor-deposited to A catalytic layer (23) is formed. Very stable carbide bonds are created at the interface by the chemical reaction of the carbide-forming species with the carbon, while an alloy phase of both is created at the interface between the carbide-forming species and the noble metal. Thus, in general, the catalytic noble metal is made to adhere very stably to the support material ( 21 ), thereby reducing degradation effects and extending the service life of the material.

Description

Method for preparing supported catalyst material for fuel cell

The present invention relates to a method for preparing a supported catalyst material for catalyzing an electrode of a fuel cell. The invention also relates to a catalyst material which can be produced by the method, to an electrode structure for a fuel cell having such a material, and to a fuel cell having such an electrode structure.

Fuel cells use the chemical conversion of fuel and oxygen into water to produce electrical energy. For this purpose, fuel cells comprise as a core component a so-called membrane-electrode-assembly (MEA, i.e. a membrane electrode assemblymembrane electrode assembly) Which is a structure composed of a membrane that conducts ions (typically, protons) and catalytic electrodes (anode and cathode) disposed one on each side of the membrane. The latter mostly contain supported noble metals, in particular platinum. In addition, Gas Diffusion Layers (GDLs) can be arranged on both sides of the membrane-electrode assembly, on the side of the electrodes facing away from the membrane. Generally, a fuel cell is formed of a plurality of stacked MEAs, the electric power of which is added. Between the individual membrane-electrode assemblies, bipolar plates (also called flow field plates or separator plates) are usually arranged, which ensure the supply of the operating medium (i.e. reactants) to the individual cells and are usually also used for cooling. In addition, the bipolar plate provides an electrically conductive contact to the membrane electrode assembly.

In the operation of the fuel cell, fuel (anode operating medium), in particular hydrogen H₂Or a hydrogen-containing gas mixture, is supplied to the anode through the anode-side open flow field of the bipolar plate, wherein H is supplied₂Electrochemical oxidation to protons H⁺And release electrons (H)₂ → 2 H⁺ + 2 e^–). The protons are transported from the anode compartment (water-bound or anhydrous) into the cathode compartment via an electrolyte or a membrane that separates the reaction chambers from one another in a gas-tight manner and is electrically insulated. Electrons provided at the anode are conducted to the cathode through a wire. Oxygen or a gas mixture containing oxygen (for example air) is supplied as a cathode operating medium to the cathode via the cathode-side open flow field of the bipolar plate, so that O is introduced₂Reduction to O by absorption of electrons^2-（½ O₂ + 2 e^– → O^2-). At the same time, the oxygen anions react in the cathode compartment with protons transported via the membrane to form water (O)^2- + 2 H⁺ → H₂O）。

For the catalysis of the above fuel cell reaction, platinum or a platinum alloy is used as the catalytic material. Since the reaction involves electrochemical surface processes, it is desirable to have as large a catalytic surface area (ECSA) as possible. For this purpose, catalytic material particles in the size range of a few nanometers are applied to a carbon support having a large surface area. However, during operation of the fuel cell, a portion of the electrical power is lost due to electrode degradation. This is mainly due to (unfavorable) operating conditions leading to loss of ECSA and activity. The underlying principle consists mainly of the dissolution of platinum from the carbon support (platinum corrosion), whereby the particles lose their electrical contact and no longer contribute to effective catalysis. In addition, agglomeration (intergrowth) of particles occurs, with a consequent reduction in catalytic surface area. Other degradation principles include corrosion of the alloying elements cobalt or nickel and corrosion of the platinum itself, growth of platinum nanoparticles by austenite ripening, growth of platinum nanoparticles on the carbon surface by migration and sintering.

In order to counteract the loss of catalytic activity and thus to be able to guarantee the performance requirements during the operating time of the fuel cell, an excess of noble metal is generally used in the preparation of the electrodes. However, this measure is very expensive.

Furthermore, it is known to use stabilized carbon supports. Although the corrosion (dissolution) of carbon is improved, the adhesion of the catalyst particles is not improved.

Furthermore, nanostructured thin film catalysts are known in which the amount of platinum can be reduced by increased service life. However, problems arise in outputting reaction water due to the nanostructure.

Furthermore, the activity of the catalyst is increased by the addition of further elements (in particular cobalt and nickel) in order to be able to ensure a higher fuel flux and thus a high electrical power. However, the problem of lack of adhesion cannot be solved by adding alloying elements. Due to the less expensive nature of these elements compared to platinum, such catalysts are even substantially more susceptible to corrosion.

There has also been research work on new catalyst support concepts where an oxide-based adhesion promoter layer should improve the adhesion of the catalyst material. However, the oxides used are generally poor conductors, resulting in power losses due to the contact resistance between the catalyst and the support.

DE 69824875T 2 describes the preparation of electrically non-conductive, nanostructured support structures on a support film from organic pigments. These support structures are coated by physical or chemical vapor deposition (PVD, CVD) to produce nanostructured catalyst elements which are then transferred directly to the polymer electrolyte membrane of the fuel cell by a stamping process. By depositing different materials sequentially, the catalyst element can have different compositions on its surface and in its internal volume.

Furthermore, mixtures of metal carbides with catalytic materials or the direct application of metal carbides to carbon supports are known (e.g. EP 1842589 a 1). Here, too, physical adhesion of the carbides to the support material is obtained.

US 2006/0183633 a1 describes a catalyst structure for the anode of a direct methanol-fuel cell (DMFC). It comprises a carrier material of Al, Ti, TiN, W, Mo or Hf, on which local elevations (nanodots) of metal carbides, such as WC, MoC or TaC, are deposited by physical or chemical vapor deposition and on which catalytic particles are deposited by physical or chemical vapor deposition. These projections as well as the catalytic particles are composed of metal carbides, such as WC, MoC or TaC, and may optionally be provided with a coating consisting of Carbon Nanohorns (CNH).

The catalytic particles are typically present on a high specific surface area support material capable of conducting electricity, wherein the support material is typically a particulate carbon-based material, such as Carbon Nanotubes (CNTs) and the like. The deposition of catalytic particles on the support material is mostly carried out by wet-chemical methods, for example by sol-gel methods using organometallic precursor compounds of the catalytic metals (for example, US 8,283,275B 2). Furthermore, it is also known to deposit catalytic noble metal particles from the vapour phase on a carbon support (e.g. US 7,303,834B 2). Subsequently, the thus supported catalyst is mixed with the ionomer and applied as a coating in the form of a slurry or suspension on a carbon paper, directly on a polymer electrolyte membrane or on a gas diffusion layer and dried.

It is an object of the present invention to provide a method of preparing a supported catalytic material for catalyzing an electrode of a fuel cell that results in a material that at least partially solves the problems of the prior art. In particular, catalyst materials are prepared which better enable the catalytic material to adhere to the support material, thereby improving stability and extending the service life.

The object is achieved by a production method having the features of the independent claims, a supported catalyst material which can be produced by the method, an electrode structure having such a catalyst material and a fuel cell having such an electrode structure. Preferred embodiments of the invention will become apparent from the further features mentioned in the dependent claims and the following description.

The process according to the invention for preparing a supported catalyst material for fuel cell electrodes comprises the following steps (in particular in the given order):

-providing a carbon-based carrier material capable of conducting electricity;

-depositing a carbide-forming substance from a vapour phase on the carbon-based support material to produce a carbide-containing layer by chemical reaction of the carbide-forming substance with the carbon of the support material, and

-depositing a catalytically active noble metal or alloy thereof from the vapour phase to form the catalytic layer.

The invention is therefore characterised in that the chemical bond between the carbide-forming substance and the support material is formed by creating a carbide or carbide-like bond between the substance and the carbon. The bonding of the catalyst material to the surface can thereby be improved. In the known processes, the catalytic material is deposited from the gas phase by wet chemistry or by purely physical deposition methods, so that only a physical bond is present by adsorption, which is clearly weaker in properties than a chemical (covalent) bond. Due to the stable binding, diffusion of the catalytic structure on the surface of the carbon support is prevented, thereby preventing their intergrowth (sintering). Carbides are also characterized by high mechanical and chemical stability and high electrical conductivity. Therefore, power loss caused by contact resistance between the catalyst and the support can be minimized.

It will be appreciated that the carbide-containing layer does not necessarily form a separate carbide phase, for example in the form of nano-crystallites with a crystalline carbide structure grown on the surface. Rather, it is sufficient to create carbide bonds or carbide-like bonds on the direct interface layer between the carbon and carbide-containing layer; that is, covalent bonds are formed between the carbide-forming elements and carbon, such that there are chemical bond conditions that correspond locally to those in the carbide crystals, but do not have the periodicity and long-range order of crystal correspondence. Furthermore, a phase of pure, unreacted carbide-forming substance, which is also part of the carbide-containing layer, can then be present on this interface layer.

Furthermore, a stable alloy between the carbide-forming substance (hereinafter also referred to as carbide former) and the noble metal or alloy thereof may be formed on the interface layer between the carbide-containing layer and the catalytic noble metal layer. Thus, stable bonding of the catalyst to the carbon support at all interfaces is achieved.

The deposition of the carbide-forming substances and the deposition of the catalytically active noble metal or alloy thereof is carried out from the gas phase, i.e. not by a wet-chemical process from a solvent. By deposition from the vapor phase, the targeted structuring can be achieved at the atomic level, which cannot be achieved by conventional wet chemical synthesis routes. Furthermore, vapor deposition can significantly reduce the amount used, particularly the amount of catalytic noble metal or alloy thereof. Thus, precious metals are used only on the surface where the catalytic reaction is taking place, while there is a lower cost material inside.

The deposition may be carried out by any vapor deposition process. Suitable methods include physical vapor deposition (physical vapor deposition, PVD), chemical vapor deposition (chemical vapor deposition, CVD), atomic layer deposition (atomic layer deposition, ALD), and the like.

The main purpose of the carbide-containing layer is to ensure stable adhesion of the noble metal or its alloys to the support material and at the same time to build up the catalytic structure in its interior by means of comparatively inexpensive materials. In order to achieve good bonding on the carbon support, a suitable lattice of carbides or carbide-forming substances is advantageous, that is to say a similar crystal structure and a similar lattice constant to the carbon material. Furthermore, in order to achieve a stable bonding of the catalytic surface layer of the noble metal, a high surface energy and a lattice structure matched to the catalytic noble metal or its alloys are also desired, wherein it is preferred here that the lattice constant of the carbide or carbide-forming substance corresponds as much as possible to the lattice constant of the catalytic surface layer, in particular of platinum. By selecting the crystal lattice of the carbide to have a slightly smaller lattice constant, a contraction of the noble metal crystal lattice is achieved, and by the resulting lattice distortion effect and also by the quantum mechanical interaction between the crystal lattices, the activity of the noble metal can be increased and a particularly dense ball packing of the noble metal can be achieved. Overall, adhesion is determined by surface energy, good matching between lattices in terms of symmetry and lattice parameters, and bond state at the interface (displacement of the d-band center). The bulk material of the core layer should also have good electrical conductivity in order to accept or release electrons during the catalytic fuel cell reaction that takes place on the catalytically active noble metal.

Suitable carbide-forming species meeting these criteria include titanium (Ti), zirconium (Zr), hafnium (Hf), tungsten (W), molybdenum (Mo), boron (B), vanadium (V), aluminum (Al), scandium (Sc), yttrium (Y), silicon (Si), chromium (Cr) and nickel (Ni) or mixtures of two or more of these elements. Among these, titanium is particularly preferred, which reacts with carbon to give titanium carbide TiC.

For vapor deposition of carbide-forming substances, the pure elements themselves or their chemical precursor compounds are used, converted into the vapor phase and deposited on the surface of the carbon-based support material. The formation of the corresponding carbide or carbide-like bond is here spontaneously caused by a chemical reaction with carbon.

The layer thickness of the carbide-containing layer is preferably selected to be as small as possible in the range of a few atomic layers relative to the carbide-forming substance. In an advantageous embodiment, the layer thickness is on average from 1 atomic layer to 50 atomic layers, in particular on average from 1 atomic layer to 20 atomic layers, preferably on average from 1 to 10 atomic layers. These layer thicknesses are sufficient to achieve the desired carbide formation.

Likewise, the layer thickness of the surface layer of the catalytically active noble metal or its alloys is preferably chosen as thin as possible in order to contact the expensive material on the surface as completely as possible. In particular, the layer thickness of the surface layer is on average 1 to 6 atomic layers, preferably 1 to 4 atomic layers, particularly preferably on average 1 to 2 atomic layers.

Adjusting the layer thickness of the individual layers within a few atomic layers makes it possible to specifically utilize the interface effect for fuel cell catalysis. By suitably selecting the duration of the vapour-depositing process, the layer thickness can be easily adjusted.

As the noble metal of the catalytic surface layer, the following platinum group metals are particularly considered: ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir) and platinum (Pt) or alloys of these metals. In particular, the surface layer comprises platinum or a platinum alloy, preferably substantially pure platinum.

In a preferred embodiment of the invention, the deposition of the carbide-forming substances and the deposition of the catalytically active noble metal or alloy thereof are carried out with time overlapping one another, whereby the deposited layer is gradually enriched from the inside outwards with the catalytically active noble metal or alloy thereof or gradually depleted in carbide-forming substances. This can be achieved by continuously or stepwise varying the relative proportions of carbide former and noble metal or alloy thereof in the gas phase during the deposition process. For example, the proportion of carbide formers may vary from 100% to 0% and the proportion of noble metals or alloys thereof may vary from 0% to 100% during the deposition process. Such adjustment of the gradient can be easily achieved using the vapor deposition process. The product is a continuous layer structure in which the content of carbide formers decreases from the inside to the outside and the content of noble metals or alloys thereof increases accordingly. Advantageously, the outermost or at least the outermost atomic layer consists 100% of the catalytic noble metal or an alloy thereof.

According to another embodiment of the method, a diffusion barrier layer is deposited on the carbide containing layer after deposition of the carbide forming species and before deposition of the catalytically active noble metal or alloy thereof. The diffusion barrier layer prevents diffusion of electrochemically less expensive materials forming the carbide former of the core, whose atoms are dissolved out by the corrosive environment in the fuel cell, on the surface, so that the catalyst becomes unstable. In a further vapour-depositing step, the diffusion barrier layer is preferably covered as completely as possible by the catalytic noble metal or alloy thereof. Suitable materials for the diffusion barrier include, for example, gold (Au), palladium (Pd), ruthenium (Ru), tungsten (W), osmium (Os), rhodium (Rh) and iridium (Ir) or mixtures or alloys of these. Gold is particularly preferred.

In another advantageous embodiment, defect sites and/or functional groups are generated on the surface of the carbon-based support material prior to depositing the carbide-forming substance. In the context of the present invention, defect sites are understood to be defects in the lattice structure of carbon, i.e. deviations from the rest of the lattice structure. Thus, the defect sites include missing positions, i.e., empty lattice positions, and the presence of heteroatoms (other than C) at lattice positions of carbon, which causes lattice distortion due to the different atomic radii of heteroatoms from C, or at lattice positions (in the form of insertions), which may also cause lattice distortion. Suitable heteroatoms include, for example, nitrogen (N), boron (B), oxygen (O), silicon (Si), and the like. Functional groups are understood to be chemical groups which are covalently bonded to the carbon of the support. These functional groups include, for example, hydrogen (-H), hydroxyl (-OH), carboxyl (-COOH), etc. This locally breaks the bonds, in particular double bonds, at the carbon surface and produces functional groups which can be used for reaction with the carbide former. The defect sites and functional groups on the surface of the carbon-based support material act as condensation nuclei for the deposition of carbide-forming substances and at the same time their increased reactivity favors the formation of chemical carbides. The resulting local formation of carbide-like compounds between the carbon and carbide formers serves as nuclei for further growth of layers or catalytic structures on the carbon support. The deposition of further atoms of the carbide former preferably takes place at this location and leads to the growth of particles. However, in principle, it is also possible to use carbon supports which have not been pretreated. This also leads to local carbide formation and growth of the structure surrounding the core.

The deletion positions can be generated by irradiating the support material with high-energy electromagnetic radiation or with particle radiation or by treatment with a gas plasma. Alternatively, a carrier material which already has defect sites can be used. The heteroatoms or functional groups can likewise be introduced by means of a gas plasma, in particular hydrogen (H)₂) Oxygen (O)₂) Nitrogen (N)₂) Etc., or introduced or generated by treatment with a chemical agent, such as an acid.

The main purpose of the carbon-based support material is to provide a large specific surface area for the applied catalyst and to create an electrical connection between the catalytic center of the material and the external circuitry of the fuel cell. Preferably, the carbon-based support material has a porous, particulate, ready-to-pile (schuttf ä hig) structure. This includes, inter alia, substantially spherical shapes or fibers. Suitable materials include, inter alia, carbon nanostructures such as carbon nanotubes, carbon nanohorns (Kohlenstoff-nanohorn), carbon nanofibers, carbon nanoribbons, carbon hollow spheres; and graphite, volcanic ash, graphitized carbon, graphene, Ketjen Black (Ketjen Black), acetylene Black (e.g., (iii))Acetylene Black) Furnace black, carbon black, activated carbon and mesophase carbon: (meso phase carbon）。

Preferably, the structure consisting of the carbide-containing layer and the catalytic layer and optionally further layers is present on the carbon-based support material in the form of a discontinuous catalytic structure. In the present invention, the term "catalytic structure" is understood to mean a formation formed (grown, deposited) on a carbon-based support material and arranged substantially discontinuously (i.e. separately from each other) on the support material. Thus, there may be gaps between adjacent catalytic structures that expose the support material. The catalytic structure can here have any shape, for example a shape approximating a spherical cross-section, in particular a hemispherical shape. It will be appreciated, however, that the structure does not generally have the ideal shape of a spherical surface and is determined in particular by the crystal structure of the material used. Regardless of their geometry, the catalytic structures have a core-shell structure, more precisely a "cut" core-shell structure, in which a "section" is arranged on and in contact with a support material.

Another aspect of the invention relates to a supported catalyst material for a fuel cell electrode (which may in particular be prepared using the method of the invention) and a carbon-based support material capable of conducting electricity, a carbide-containing layer deposited on the support material and a catalytic layer of a catalytically active noble metal or an alloy thereof deposited on the surface of the carbide-containing layer.

Said materials are characterized by excellent adhesion of the catalytic layers and thus low corrosion and sintering tendencies. Furthermore, relatively small amounts of precious metals or alloys thereof are required, since the interior of the catalytic structure is substantially filled with the less expensive carbide formers.

Another aspect of the invention relates to an electrode structure for a fuel cell comprising a planar support and a catalytic coating arranged on at least one of the two surface sides of the support, which catalytic coating comprises a supported catalyst material according to the invention. Here, the planar support is, for example, a polymer electrolyte membrane for a fuel cell. In this case, it is also referred to as a Catalytically Coated Membrane (CCM). Alternatively, the flat support can be a gas-permeable, electrically conductive substrate, for example a Gas Diffusion Layer (GDL) or another support layer of the fuel cell, for example carbon paper or the like. In the case of a catalytically coated gas diffusion layer, this is also referred to as a gas diffusion electrode.

The electrode structure can be produced by coating the catalyst material directly on a flat support. For this purpose, a suspension or slurry is prepared which comprises the catalyst material, the solvent and optionally further additives, such as binders and the like, and is applied to a flat support by any desired method and dried.

Another aspect of the invention relates to a fuel cell having a polymer electrolyte membrane and at least one of the layers having the supported catalyst material of the invention respectively disposed on the surface sides thereof.

The various embodiments of the invention mentioned in the present application can be advantageously combined with one another, unless otherwise stated in individual cases.

The invention will be elucidated in a specific embodiment with reference to the drawings. Wherein:

FIG. 1 is a schematic illustration of a supported catalyst material according to a first embodiment of the invention;

FIG. 2 is a schematic illustration of a supported catalyst material according to a second embodiment of the invention;

FIG. 3 is a schematic illustration of a supported catalyst material according to a third embodiment of the present invention;

FIG. 4 is a schematic illustration of a supported catalyst material according to a fourth embodiment of the present invention;

FIG. 5 is a schematic illustration of a supported catalyst material according to a fifth embodiment of the present invention;

FIG. 6 is a schematic illustration of a supported catalyst material at the molecular level according to a fifth embodiment of the present invention;

FIG. 7 is a schematic illustration of a supported catalyst material according to a sixth embodiment of the invention; and

fig. 8 is a cross-sectional view of a fuel cell having a catalyst material according to the present invention.

Figures 1 to 7 show highly schematically and idealistically a supported catalyst material according to the present invention. The material sections shown are each very greatly exaggerated, as in the form of a section through an exemplary structure. Here, the same reference numerals are used for identical elements and are not specifically set forth for each embodiment.

The supported catalyst material, generally designated 20, according to fig. 1 has a carbon-based support material 21 which is capable of conducting electricity and is selected from the materials mentioned above, here for example carbon black. A deposited carbide-containing layer 22 is arranged on the support material 21, which layer is selected from one of the materials mentioned above, here for example titanium Ti. At least at the interface between the support material 21 and the carbide-containing layer 22 is carbide, here titanium carbide TiC, or forms a carbide-like bond, i.e. here a chemical bond, in particular a covalent bond, is formed between the two

layers

21, 22. A catalytic layer 23 of a catalytically active noble metal selected from one of the above-mentioned materials, here for example platinum Pt, or an alloy thereof, is deposited on the surface of the carbide-containing layer 22. An alloy of the carbide former Ti and the noble metal Pt forms at the interface between the carbide-containing layer 22 and the catalytic layer 23, so that here too closely bound (stoffschlussig) compounds are present. The structure formed by the carbide-containing layer 22 and the catalytic layer 23 is present in the form of a plurality of discrete catalytic structures or particles, only one of which is shown here.

The catalyst material according to fig. 2 differs from the catalyst material in fig. 1 in that catalytic structures are grown on both sides of the support material 21. Instead of the symmetrical arrangement shown, the structures can also be arranged asymmetrically on both sides of the carrier material 21.

The catalyst material 20 according to fig. 1 and 2 can be prepared by: a carbide former Ti is first deposited on the carbon-based support material 21 by means of a vapour deposition process to form a carbide containing layer 22 having a desired layer thickness. Here, each titanium atom initially deposits and forms carbides locally, and the layer 22 grows by the accumulation of other titanium atoms on the "core". After the desired layer thickness of, for example, 1 to 10 atomic layers Ti is reached, noble metal Pt is then deposited on the carbide-containing layer 22 by another vapor deposition process to form the catalytic layer 23. After the desired layer thickness of, for example, 1 to 5 atomic layers Pt is reached, the catalyst material 20 is obtained. Whether the catalytic structure is grown on one or both sides of the support material 21 depends mainly on the accessibility of the different sides of the support material.

The catalyst material 20 shown in fig. 3 differs from the catalyst material in fig. 1 in that the carbon-based support material 21 has defect sites 24. Here, lattice defects in the crystal lattice of carbon are involved in this embodiment.

The catalyst material 20 shown in fig. 4 substantially corresponds to the catalyst material in fig. 3, with the difference that the catalytic structure is grown on both sides of the carbon support 21 similar to that shown in fig. 2.

The catalyst material 20 according to fig. 3 and 4 may be prepared by a method similar to that used for the specific embodiment according to fig. 1 and 2, with the difference that the carbon support material 21 is subjected to a treatment process, for example a plasma treatment, prior to the deposition process of the carbide formers to produce defect sites 24. The resulting defect sites 24 facilitate the initial deposition and nucleation of carbide formations.

Fig. 5 and 6 show another embodiment of a catalyst material 20 according to the present invention. In contrast to the materials according to fig. 1 to 4, a mixed layer 25 is present between the carbide-containing layer 22 and the catalytic layer 23, which mixed layer consists of a mixture or alloy of carbide formers and catalytic noble metals, in this case a Pt — Ti alloy. Specifically, the mixed layer 25 is formed such that the content of Ti decreases from the inside to the outside, and the content of Pt conversely increases. Thus, the

entire layer structure

22, 25, 23 can also be described as a single continuous layer, in which the titanium content decreases from 100% (inner) to 0% (outer surface) and the platinum content increases from 0% (inner) to 100% on the surface. According to another embodiment, the mixed layer 25 has a homogeneous Pt-Ti-alloy, i.e. no concentration gradient. Furthermore, it is also possible to grow the layer structure according to fig. 5 and 6 on both sides of the carbon carrier 21 (analogously to fig. 2 and 3) and/or to generate defect sites 24 in the carbon carrier 21 in combination (analogously to fig. 3 and 4).

The material 20 according to fig. 5 and 6 can be prepared by continuously changing the composition of the gas phase in a continuous gas phase deposition process.

Another embodiment of a catalyst material 20 according to the present invention is shown in fig. 7. In contrast to the materials according to fig. 1 to 4, a diffusion barrier layer 26 is present here between the carbide-containing layer 22 and the catalytic layer 23, which diffusion barrier layer is composed of one of the materials mentioned above, here, for example, gold Au. The diffusion barrier 26 prevents the less noble carbide former from diffusing onto the surface of the catalytic structure and thus from leaching out. The material 20 according to fig. 7 may be prepared by depositing the material of the diffusion barrier layer 26 on the layer 22 and then depositing the catalytic noble metal on the diffusion barrier layer 26 in a further vapour deposition step after the deposition of the carbide former.

To produce the electrodes of the fuel cell, a composition (suspension, slurry, etc.) is first prepared from the catalytic material 20 according to the invention, which composition comprises a solvent in addition to the catalytic material 20 and may comprise further additives, in particular a polymer binder. The composition is then applied as a coating to a flat support, for which purpose any coating method can be used, for example brushing, spraying, knife coating, printing, etc. The flat support is in particular a polymer electrolyte membrane of a fuel cell, which is preferably coated on both sides with a catalytic material. Alternatively, the catalytic coating may also be applied on the gas diffusion layer or on another gas-permeable, electrically conductive substrate, for example carbon paper.

Fig. 8 shows the structure of such a fuel cell 10 in a schematic sectional view. At the heart of the fuel cell 10 is a membrane-electrode-assembly (MEA), generally designated 14. The MEA 14 includes a polymer electrolyte membrane 11, two catalytic electrodes or catalytic coatings (i.e., an anode 12a and a cathode 12 k) disposed on the surface side thereof, and then two gas diffusion layers 13 disposed on both sides. The polymer electrolyte membrane 11 is an ion-conducting (in particular proton-conducting) polymer, for example under the trade name Nafion^®The product for sale. The

catalytic electrodes

12a, 12k contain the catalytic material according to the invention and are constructed as a two-sided coating of the membrane 11 in the embodiment shown. The gas diffusion layer 13 is made of a gas-permeable, electrically conductive material, which has a structure of foam or fiber structure, for example, and serves to distribute the reaction gas on the

electrodes

12a and 12 k. On both sides of the membrane-electrode assembly 14 are then bipolar plates 15, namely an anode plate 15a and a cathode plate 15 k. Typically, a plurality of such single cells 10 are stacked to form a fuel cell stack, and thus each bipolar plate is composed of an anode plate 15a and a cathode plate 15 k. The

bipolar plates

15a, 15k each comprise a structure of reactant channels 16 which are open in the direction of the gas diffusion layer 13 and serve to supply and distribute the reactants of the fuel cell. Thus, fuel, here hydrogen H, is supplied to the anode plate 15a via the reactant passage 16₂And oxygen O is supplied to the cathode plate 15k via the corresponding channel 16₂Or a gas mixture comprising oxygen, in particular air. The

bipolar plates

15a, 15k are connected to one another via an external circuit 18 and to an electrical consumer 19 (for example a traction motor or a battery for an electric vehicle).

During operation of the fuel cell 10, hydrogen is supplied to the anode plate 15a via the reactant channels 16, via the gas on the anode sideThe diffusion layer 13 is distributed and supplied to the catalytic anode 12 a. Here, hydrogen H₂Catalytic dissociation and oxidation to protons H⁺As the electrons are released, they are output via circuit 18. On the other hand, oxygen is transferred to the catalytic cathode 12k through the gas diffusion layer 13 on the cathode side via the cathode plate 15 k. At the same time, protons H formed on the anode side⁺And diffuses to the cathode 12k via the polymer electrolyte membrane 11. Here, the supplied air oxygen gas reacts with protons on the catalytic noble metal by receiving electrons supplied via the external circuit 18 to obtain water, which is output from the fuel cell 10 together with the reaction gas. The electrical load 19 can be supplied with current as a result.

The catalyst material 20 according to the present invention may be used in the anode 12a and/or the cathode 12k of a fuel cell. The fuel cell 10 equipped with the catalytic material 20 according to the invention is characterized in that the

catalytic electrodes

12a, 12k have a low tendency to corrosion and therefore a high long-term stability. At the same time, relatively less catalytic precious metal is required since the main volume of the catalytic material of the electrode is formed of a relatively inexpensive material.

List of reference numerals

10 fuel cell

11 polymer electrolyte membrane

12 catalytic electrode

12a anode

12k cathode

13 gas diffusion layer

14. Membrane-electrode-assembly

15 Bipolar plate

15a anode plate

15k cathode plate

16 reactant channels

17 coolant channels

18 circuit

19 consumer/electric load

20 supported catalyst material

21-carbon-based carrier material

22 carbide-containing layer

23 catalyst layer

24 defect location

25 mixing layer

26 diffusion barrier layer

Claims

1. A method for preparing a supported catalyst material (20) for a fuel cell electrode, comprising the following steps:

- providing a carbon-based support material capable of conducting electricity (21);

- vapor deposition of carbide-forming substances on said carbon-based support material (21), whereby a carbide-containing layer (22) is produced by chemical reaction of the carbide-forming substances with the carbon of the support material (21), and

- vapor deposition of catalytically active noble metals or their alloys to form catalytic layers (23),

in,

The deposition of the carbide-forming substance and the deposition of the catalytically active noble metal or alloy thereof are carried out overlapping in time, so that the catalytically active noble metal or alloy thereof is gradually enriched in the deposited layer; or

After depositing the carbide-forming species and before depositing the catalytically active noble metal or alloys thereof, a diffusion barrier (26) is deposited selected from the group consisting of gold (Au), palladium (Pd), ruthenium (Ru), tungsten ( W), osmium (Os), rhodium (Rh) and iridium (Ir) or their mixtures or alloys.

2. The method according to claim 1, wherein the carbide-forming substance is selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), tungsten (W), molybdenum (Mo), boron ( B), vanadium (V), aluminium (Al), scandium (Sc), yttrium (Y), silicon (Si), chromium (Cr) and nickel (Ni) or mixtures of these.

3. The method according to claim 1 or 2, characterized in that the layer thickness of the carbide-containing layer (22) is in the range of 1 to 50 atomic layers.

4. The method according to claim 1 or 2, characterized in that the layer thickness of the carbide-containing layer (22) is in the range of 1 to 20 atomic layers.

5. The method according to claim 1 or 2, wherein the catalytically active noble metal or its alloy comprises platinum (Pt), ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os) ), iridium (Ir), or alloys of these metals.

6. The method of claim 1 or 2, wherein the catalytically active noble metal or alloy thereof comprises platinum (Pt).

7. The method according to claim 1 or 2, characterized in that defect sites and/or functionalizations (24) are created on the surface of the carbon-based support material (21) prior to depositing the carbide-forming species.

8. The method according to claim 1 or 2, characterized in that prior to depositing the carbide-forming substance, the carbon-based support material is deposited on the carbon-based support material by treatment with gas plasma, with chemical agents, with particle radiation or with electromagnetic radiation Defect sites and/or functionalization (24) are created on the surface of (21).

9. The method according to claim 1 or 2, characterized in that the structure consisting of the carbide-containing layer (22) and the catalytic layer (23) is present on the carbon-based support material in the form of a discontinuous catalytic structure.

10. A supported catalyst material (20) for fuel cell electrodes (12a, 12k) prepared according to any one of claims 1 to 9, comprising an electrically conductive carbon-based support material (21) deposited on the support A carbide-containing layer (22) on the material (21) and a catalytic layer (23) of a catalytically active noble metal or alloy thereof deposited on the surface of the carbide-containing layer (22).

11. An electrode structure for a fuel cell (10), comprising a planar carrier (11, 13) selected from the group consisting of a polymer electrolyte membrane (11) and a gas-permeable conductive substrate (13), and arranged on the carrier (11) , 13) on at least one surface side of a catalytic coating (12a, 12k) comprising the supported catalyst material (20) according to claim 10.